I would go with all 15" wheels.  Funny though, as long as the axles have the
same size on both sides, it should not make it handle that badly.

Are you sure your shocks are good/  The ride on a split is supposed to be
tight,  not sqishy like a car or a later bus.  Hows the play in the steering.
 You could jack it up and check wear in the kingpins (wobble when you push
pull at 12 and 6 o clock)  and tie rods etc (wobble from 3 to 9 oclock.)

just a thought

clara